It is becoming clear that many, if not most, malignancies arise from a population of cells that exclusively maintain the ability to self-renew and sustain the tumor via the expression of tumor progenitor genes. These “cancer stem cells” are often biologically distinct from the differentiated cancer cells that comprise most of the tumor bulk. Because cancer stem cells are believed to be primarily responsible for tumor initiation as well as resistance to chemo- and radiotherapy, their persistence may account for relapsing disease in cancer patients.
Wilms' tumor, a type of kidney cancer or nephroblastoma is one of the most common solid tumors of childhood, occurring in 1 in 10,000 children and accounting for 8% of childhood cancers. With improved multimodality therapy, WT survival rates have risen over the last 40 years to 85%-90%; however, for those whose disease relapses or metastasizes, even intensive salvage regimens result in subsequent survival closer to 50%. Moreover, survivors are at increased risk for a broad spectrum of adverse outcomes caused by chemotherapy and radiation therapy, such as late mortality and secondary cancers.
Wilms tumor is believed to result from malignant transformation of abnormally persistent renal stem cells which retain embryonic differentiation potential. Indeed, recent molecular data obtained by microarrays demonstrate that Wilms' tumor and Wilms' tumor-stem like xenografts systematically overexpress nephric-progenitor genes corresponding to the earliest stages of normal metanephric kidney development connecting tumorigenesis and organogenesis in the kidney.
In addition to genes that specify the renal lineage, Wnt pathway-related molecules including β-catenin (CTNNB1), frizzled7 (FZD7) and frizzled2 (FZD2), are concomitantly induced in Wilms' tumor. In general, the binding of Wnt ligand to frizzled cell surface receptors normally leads to inhibition of a “destruction complex” consisting of APC/Axin/GSK-3β/Ckl/Dvl and other factors, with subsequent accumulation of dephosphorylated stabilized β-catenin, and transcription of its target genes. Interestingly, recent data have demonstrated an essential role for the Wnt-β-catenin signaling pathway in nephrogenesis. Also, a genomic screen of Wilms' tumor identified FZD7, a Wnt receptor, to be a biomarker of cancer stem cells in Wilms' tumor. Additionally, a striking link between β-catenin signaling and the development of Wilms tumor has been demonstrated.
In recent years the involvement of the Wnt pathway has been shown in various other tumors (e.g. colon, breast) and in the self-renewal mechanism of stem cells. Unregulated activation of the Wnt can alter the developmental fate of tumor cells to maintain them in an undifferentiated and proliferative state. Thus carcinogenesis can proceed in the context of altered homeostatic mechanisms controlling normal development and tissue repair by stem cells.
Secreted Wnt ligands activate a receptor complex consisting of a Fzd receptor family member and low-density lipoprotein (LDL) receptor-related protein 5 or 6 (LPR5/6). The Fzd receptors are seven transmembrane domain receptors of the G-protein coupled receptor (GPCR) superfamily and contain a large extracellular N-terminal ligand binding domain with 10 conserved cysteines, known as the cysteine rich domain. There are ten known human FZD receptors: FZD1-10. Different Fzd cysteine rich domains have different binding affinities for specific Wnts. Fat receptors have been grouped into those that activate the canonical β-catenin pathway and those that activate noncanonical pathways.
To form the receptor complex that binds the FZD ligands, FZD receptors interact with LRP5/6, single pass transmembrane proteins. The canonical Wnt signaling pathway activated upon receptor binding is mediated by the cytoplasmic protein Dishevelled (Dsh) interacting directly with the Fzd receptor and results in the cytoplasmic stabilization and accumulation of β-catenin. In the absence of a Wnt signal, β-catenin is localized to a cytoplasmic destruction complex that includes the tumor suppressor proteins adenomatous polyposis coli (APC) and Axin. Activation of Dsh results in the dissociation of the destruction complex.
In addition to the canonical signaling pathway, Wnt ligands also activate β-catenin—independent pathways (i.e. non-canonical Wnt signaling). Non-canonical Wnt signaling has been implicated in the process of gastrulation. Antagonism is often reported between the canonical and non-canonical pathways, and some evidence indicates that non-canonical signaling can suppress cancer formation. Thus, in certain contexts, Fzd receptors act as negative regulators of the canonical Wnt signaling pathway. For example, FZD6 represses Wnt canonical signaling when co-expressed with FZD1.
The canonical Wnt signaling pathway also plays a central role in the maintenance of stem cell populations in the small intestine and colon, and the inappropriate activation of this pathway plays a prominent role in colorectal cancers; colorectal cancer is most commonly initiated by activating mutations in the Wnt signaling cascade. Approximately 5-10% of all colorectal cancers are hereditary with one of the main forms being familial adenomatous polyposis (FAP), an autosomal dominant disease in which about 80% of affected individuals contain a germline mutation in the adenomatous polyposis coli (APC) gene. Mutations have also been identified in other Wnt pathway components including Axin and β-catenin.
In human breast cancer, β-catenin accumulation implicates activated Wnt signaling in over 50% of carcinomas, and though specific mutations have not been identified, upregulation of Frizzled receptor expression has been observed.
Thus, there remains a need to identify molecules that may halt the inappropriate activation of the Wnt signaling pathway in cancers, including but not limited to colorectal cancer, melanomas and breast cancer. The present invention addresses this need by providing an antibody specific to the FZD7 receptor.